Kilogram facts for kids

A computer image of the International Prototype Kilogram. It is the kilogram. It sits next to an inch-based ruler for scale. Like the other prototypes, the edges of the IPK have a four-angle chamfer to minimize wear (although only three can be seen in this image).

The kilogram or kilogramme is a metric unit of mass. The official kilogram is the mass of one piece of platinum-iridium metal kept in Paris. The piece of metal is known as Prototype Kilogram (IPK). It is now the only metric unit which is defined as an object.

There are attempts to define the kilogram in other ways. One example specifies a number of atoms of a certain substance (at a certain temperature).

One kilogram is a little more than 2.2 pounds. One tonne is one thousand kilograms. One litre of water weighs almost exactly one kilogram, at 3.98° centigrade, at sea level. This was the basis of the definition of the gram in 1795.

Contents

History

In 1879, the piece of metal was made. It was officially chosen to be the kilogram in 1889. It was made of 90% platinum and 10% iridium. Those metals were chosen because they do not rust or corrode like most metals. It is stored in a vault at the BIPM in Sèvres, France. From 1795 to 1799, the unit of mass was not called "kilogram" but was called "grave".

The original kilogram is kept inside bell jars. Over time, dust can collect on it. Before it is measured, it is cleaned to get the original size.

Kilograms and mass

The chains on the swing hold the child’s weight. If one were to stand behind her and try to stop her, one would be acting against her inertia. This inertia comes from her mass, not weight.

The kilogram is a unit of mass. In normal language, measuring mass defines how heavy is something. This is not scientifically correct. Mass is an inertial property. It measures the tendency of an object to stay at a given speed when no force acts on it.

The weight of matter depends on the strength of gravity. The mass of matter does not. The mass of an object is the same everywhere. Objects are "weightless" for astronauts in microgravity. However, the objects still have their mass and inertia. Astronaut must use ten times as much force to accelerate a ten-kilogram object at the same rate as a one-kilogram object.

A common swing, as shown in the picture, can show the relationship of force, mass and acceleration. Someone could push an adult on the swing. The adult would accelerate slowly. They would only swing a short distance forward before the swing would change direction. If a child is sitting on the swing, then the child would swing forward faster and further.

Problems

There are various copies of the original Kilogram. Some of these copies have gained or lost some mass. This is a problem for scientists and engineers who need to make exact measurements.

Images for kids

The Arago kilogram, an exact copy of the "Kilogramme des Archives" commissioned in 1821 by the US under supervision of French physicist François Arago that served as the US's first kilogram standard of mass until 1889, when the US converted to primary metric standards and received its current kilogram prototypes, K4 and K20.

A replica of the prototype kilogram on display at Cité des Sciences et de l'Industrie, featuring the protective double glass bell.

National prototype kilogram K20, one of two prototypes stored at the US National Institute of Standards and Technology in Gaithersburg, Maryland, which serve as primary standards for defining all units of mass and weight in the United States. This is a replica for public display, shown as it is normally stored, under two bell jars.

The NIST's watt balance is a project of the US Government to develop an "electronic kilogram". The vacuum chamber dome, which lowers over the entire apparatus, is visible at top.

The local gravitational acceleration g is measured with exceptional precision with the help of a laser interferometer. The laser's pattern of interference fringes—the dark and light bands above—blooms at an ever-faster rate as a free-falling corner reflector drops inside an absolute gravimeter. The pattern's frequency sweep is timed by an atomic clock.

A magnet floating above a superconductor bathed in liquid nitrogen demonstrates perfect diamagnetic levitation via the Meissner effect. Experiments with an ampere-based definition of the kilogram flipped this arrangement upside-down: an electric field accelerated a superconducting test mass supported by fixed magnets.